Selective call receivers (also known as pagers) have been available for several years and the technology used therein is well known. In its basic form, a selective call receiver is a miniature radio receiver that can be carried on a user's person for receiving radio transmitted messages. Each selective call receiver has a unique address, and when that unique address is received, the selective call receiver alerts the user of a following received message. Selective call receivers, then, allow the user carrying the selective call receiver to receive a page providing that user with a message or notice while the user is away from an office or phone. The page may be a simple beep for alerting the user to call a predetermined telephone number for a message. Alternatively, the page may be more complex, for example, a telephone number or an alphanumeric message displayed on the selective call receiver's message display, or stock quotes or news information received from a news and information service for down-loading into a personal computer from the selective call receiver, etc. Selective call receivers also typically include a variety of accessories. For example, an acknowledge circuit might be included on the selective call receiver for sending a signal back to a transmitter to verify that a page has been received. Additionally, selective call receivers may use a speaker, a flashing LED (light emitting diode), and/or a tactile alert such as a vibrator to alert the user of a received page. Additional features can include backlit displays, improved antennas, low battery alert, etc.
Competition in the selective call receiver market demands not only that additional features be provided, but further that the selective call receivers maintain small form factors, while providing improved quality. Selective call receivers, for example, are now offered in small form factors that can unobtrusively be carried on a belt, on a wrist as a wristwatch pager, or in a pocket in a thin form factor roughly the size of a credit card (credit card or thin form factor). An important factor contributing to the reliability and, hence the quality, of a selective call receiver, is the accuracy of the internal clock, for example, in the controller. Crystals are available for providing accurate oscillator frequencies or clocks and have been used extensively in the past. Crystals continue to be used for providing a reference frequency to clock circuits, but the accuracy of the clock circuits themselves continue to be critical.
A variety of phase shift oscillators, phase locked loops, and other filter and comparator combinations have been made available for integration onto integrated circuits, for example, on a controller, for providing a controlled frequency or clock. Not only do such circuits need to be integratable on such a controller, but a variety of other demands are placed on such circuits, including high stability, precision, and a necessary wide operating range. Such demands are exacerbated by environmental, processing, and other factors.
Thus, what is needed is a variable frequency oscillator having an improved operational transconductance amplifier for providing an integratable, highly stable and accurate clock.